Vehicle suspension with independent pitch and roll control

ABSTRACT

A vehicle hydropneumatic suspension comprising four double acting rams (1, 2, 3, 4) each between respective one of four spaced wheels at comers of the vehicle. First conduits (9, 11) connecting main chambers (1a, 2a) of front rams (1, 2) with diagonally opposite rear cylinder minor chambers (3b, 4b), respectively, and, second conduits (10, 12) connecting minor chambers (1b, 2b) with main chambers of diagonally opposite rams (3a, 4a), respectively. A load distribution unit (13) has two cylindrical chambers (13a, 13b) each separated by a piston into. respectively, chambers (14, 15) and (16, 17). Conduits (9a, 10a, 11a, 12a) connect respectively conduits (9, 10, 11, 12) to chambers (14, 16, 17, 15). The pistons are connected by a resilient means (20) to allow relative piston movement to provide for independent control of pitch and roll of the vehicle.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to improvements in the suspension system or avehicle, and is specifically related to controlling the disposition ofthe vehicle body relative to the ground when the vehicle is subject tovariations in the contour of the surface being traversed.

Description of the Background Art

In recent times, there has been a trend towards resilient sprungsuspension systems incorporating variable damping and spring rates in anattempt to improve vehicle stability and reduce movement of the vehiclebody relative to the surface being traversed.

A range of suspension systems known as `active` and `semi-active`suspensions for vehicles have been trialed including systems operatingon the basis of compression and/or displacement of fluids, and suchsystems currently in use incorporate a pump, to maintain the workingfluid at the required pressure and effect the high speed distributionthereof, and sophisticated control mechanisms to regulate the operationof the suspension system in accordance with sensed road and/or vehicleoperating conditions. These known systems incorporating pumps andelectronic control systems, which both usually operate continuouslywhile the vehicle is in operation, are comparatively expensive toconstruct and maintain, and require a substantial energy input. Theytherefore are finding limited acceptability in the vehicle industry.

There is previously published an International Patent Application(International Publication Number WO 93/01948, International ApplicationNumber PCT/AU92/00362 and dated Feb. 4, 1993) which discloses a`passive` hydropneumatic vehicular suspension system. This disclosedpassive suspension system has many of the advantages of `active` or`semi-active` suspension systems, whilst avoiding the complexity andexpense of such systems, thereby making it more acceptable to theautomotive industry.

In the suspension system disclosed in said patent, a front wheel ram andthe diagonally opposite rear wheel ram have the upper chamber of thefront ram interconnected with the lower chamber of the rear ram and thelower chamber of the front ram interconnected to the upper chamber ofthe rear ram. Similarly the respective chambers of the other front ramand rear ram are likewise interconnected. There is thus provided twoindividual fluid circuits, each comprising a front ram and a diagonallyopposite rear ram. Each of the conduits interconnecting the respectiveupper and lower chambers normally has at least one conventional pressureaccumulator in communication therewith. The two circuits areinterconnected to a pressure balancing device which is arranged tomaintain a substantially equal pressure in the two circuits, as isdescribed in detail in the previously referred to International PatentApplication No. WO93/01948.

This prior proposed vehicle suspension system obviates the use ofordinary springs (eg. coils, leaf, or torsion bar springs) as well asconventional telescopic dampers (commonly referred to as shockabsorbers) and roll or sway stabiliser bars.

Springing or resilience is provided by way of the gas filledaccumulators with damper valves located in the mouths of theaccumulators. Conventional vehicles fitted with accumulator springs areknown to provide good comfort levels when traversing low amplitudeground surfaces at most speeds. However, accumulators gassed to providea soft ride also tend to induce and exaggerate unwanted roll and pitchmotions when used without roll or sway stabiliser bars. Mosthydropneumatically suspended vehicles are therefore normally providedwith roll or sway bars made of spring steel which mechanically andtransversely interconnect the two wheels of each axle thereby limitingroll but not pitch movements.

In the suspension system described above, (Patent # WO 93/01948),excessive roll movements are resisted and controlled hydropneumaticallywithout roll stabiliser bars and the amount of roll permitted is definedby a function of the ratio of the rams' cylindrical bore diameters (ofthe diagonally opposite rams) to the ram rod diameters, and with regardto their stroke lengths and with regard to the amount of gas within thevarious accumulators of the suspension system.

It is also to be noted that the type of wheel geometry and the locationand design of various components may give some components a mechanicaladvantage over others thereby providing for example, an appropriate butdifferent amount of roll stiffness at the front relative to the rear ofthe vehicle which to an extent defines whether the vehicles under oroversteers when cornering.

In conventional vehicles, roll forces are resisted by the roll or swaybars, i.e. transverse mounted, formed spring steel bars which must bedeformed in torsion for any body roll to occur. Conversely, pitch motionin the longitudinal plane is normally only partially resisted by thedesign of the suspension geometry with spring resonances being avoidedthrough the appropriate selection of front and rear spring and damperrates without the need for any direct acting mechanical equivalent ofthe roll bar. This is because the pitching actions in the longitudinaldirection are less severe than the transverse rolling actions.

It has been found that the system previously disclosed provides adequatecomfort, stability and relatively consistent wheel loading irrespectiveof relative wheel travel positions during many manoeuvres such as axlearticulations and single wheel inputs, however, the magnitude of pitchand roll control is governed by the same components and the effectivelinear stiffness of each wheel in relation to the vehicle body in eitherpitch or roll is typically the same. In long wheel based vehicles thistranslates to stiff pitch characteristics in relation to roll. In shortwheel based vehicles, pitch and roll stiffness become closer inmagnitude. As most vehicles are considerably narrower than they arelong, and due to other geometric effects it has been found that roll ismore difficult to control than pitch as noted above. Indeed, when thesuspension system is designed to adequately contain roll movement thepitch motions may be consequently over compensated for in the system.This may be further clarified as follows:

In order to contain high roll forces resulting from a high centre ofgravity with respect to the relatively closely located rams (in thetransverse direction), it is necessary to supply rams with a greaterdifference in rod and bore diameters. This therefore may automaticallygenerate an unnecessary amount of pitch resistance or control in thelongitudinal direction and this can lead to harshness of ride quality insome conditions. In particular it has been found that while bodydisturbance due to axle articulation movements and single wheel inputsis minimised, road surfaces that give rise to double wheel inputs on asingle axle (such as `speed humps`) or sinusoidal road profiles canupset the previously disclosed suspension system. Typically this occurswhen a vehicle's wheel base length approximates to half of the spacingof the humps disposed along the road surface. In order to traverse thiskind of road surface smoothly (without excessive pitch motions beinginduced) both axles need to become independent in their motion, howeverthe previously disclosed interrelated hydropneumatic system interpretsthese motions as high speed pitch movements and therefore attempts toresist them as though they were unwanted pitch motions. This type ofhigh speed pitch resistance and over compensation manifests itself as aninappropriate pitch harshness which can become additionallyuncomfortable when the vehicle moves over repeated bumps or dips causingincreasingly exaggerated and inappropriate resonant responses.

SUMMARY OF THE INVENTION

It is therefore the object of this invention to provide a vehiclesuspension system that will provide a more optimal relationship betweenthe pitch and roll control of the vehicle.

A suspension system for a vehicle body having a plurality of wheelsarranged in lateral and longitudinal spaced relation to support thevehicle body, said suspension system comprising individual ram meansarranged between each wheel and the vehicle body, each ram meanscomprising a cylinder, piston and rod,

a first and second balance means each having two chambers and forcetransfer means separating the chambers and displaceable in response topressure conditions in the respective chambers,

said force transfer means of each balance means being operablyinterconnected to transfer force therebetween to achieve a balancedstate between the two transfer means;

the two chambers of each balance means including an inner chamberadjacent the interconnection and an opposing outer chamber;

the inner chambers of each balance means being in fluid communicationrespectively with the ram means on one end of the vehicle, and the outerchambers of each balance means being in fluid communication respectivelywith the ram means on the opposite end of the vehicle body, such thatboth chambers of the first balance means are in fluid communication withthe ram means of one side of the vehicle body, both chambers of thesecond balance means being in fluid communication with the ram means onthe opposing side of the vehicle body;

said interconnection between said force transfer means being adapted totransfer said force and allow relative movement between said forcetransfer means to provide additional resilience in a pitch direction ofthe vehicle body relative to a roll direction of the vehicle body.

The above described suspension system has the ability to generallymaintain all wheels in tractive contact with the surface beingtraversed, particularly in situations of extreme surface irregularity asexperienced in off-road operation. In addition, effective control ofbounce pitch and roll of the vehicle is achieved by virtue of the fluidsystem controlling concurrently unidirectional movement of any twolongitudinally adjacent or two laterally adjacent wheels relative to thevehicle. Maintenance of tractive contact of all wheels with the groundis achieved by the pressure conditions in the respective rams and thecontrol of pitch and roll is by the pressure and movement of fluidbetween the rams and between the rams and the balance means.

Preferably the first and second balance means comprises first and secondcontrol chambers each divided into two cavities by respective movablewalls. The two cavities of the first control chamber communicaterespectively with the rams of the front and rear wheels on one side ofthe vehicle, and the cavities of the second control chamber communicaterespectively with the fluid cylinders of the front and rear wheels onthe opposite side of the vehicle. The force transfer means beingarranged to interconnect said movable walls to transfer forcetherebetween to achieve a balanced state between the movable walls ofthe respective control chambers. Also, the interconnection between theforce transfer means includes resilient means adapted to transfer saidforce and allow relative movement between the movable walls to achievethe balance between the net forces on the respective movable walls.

Conveniently said resilient means is arranged to be capable oftransferring both tension and compressive forces. Preferably saidresilient means is a metal or gas spring or a member of resilientmaterial, such as a member of rubber or plastic.

Conveniently one or each movable wall can be in a form that isresiliently deformable so that the total volume of the control chamberoccupied by the fluid can vary in response to the pressure in the twocavities of the respective chambers. The inclusion of a resilient meansas part of each movable wall in addition to the resilient meansinterconnecting each movable wall allows for adjustment of both pitchand roll characteristics individually and independently.

The hydraulic rams may be of either the double or single acting type. Ineither arrangement, the chambers of the rams that are providing thesupport for the vehicle are connected to the balance means. If thebalance means are to restrict the lateral roll of the vehicle whilststill permitting a degree of pitch resilience, then the chambers of thefirst balance means are in communication with the fluid chambers of thewheels on one longitudinal side of the vehicle and the chambers of thesecond balance means are in communication with the fluid cylinders ofthe wheels on the other side of the vehicle.

If the pitch motion of the vehicle is the dominant factor to berestricted, while still allowing some additional resilience about theroll axis, then the fluid cylinders of the front wheels are incommunication with the first balance chamber and the fluid cylinders ofthe rear wheels in communication with the second balance chamber.

It will be appreciated that the provision of the resilient meansinterconnecting the movable walls in the respective control chambersenables at least a part of the movement of one wall to be absorbed bythe resilient means therebetween so that a different degree of movementis transferred to the other movable wall. The resilient means can beeither resiliently elongated or compressed and thus the difference inthe extent of movement of the respective movable walls can be by way ofan increase or decrease. The effect of this differential in the extentof movement of the respective movable walls is that a lesser degree ofmovement is transferred to the other wheels in response to a severe orrapid movement of both wheels on one axle of the vehicle in the samedirection is that a lesser degree of movement is transferred to theother wheels, thus reducing or even reversing the aforementionedundesirable pitch control characteristics. When movable walls or membersare constructed incorporating resilient means the other types ofresilient means such as accumulator may be omitted.

A suspension system for a vehicle having a load support body, and a pairof front ground engaging wheels and a pair of rear ground engagingwheels connected to the body to support same and each wheel beingdisplaceable relative to the body in a generally vertical direction, thesuspension system comprising a double acting ram interconnected betweeneach wheel and the body, each ram including first and second fluidfilled chambers varying in volume in response to relative verticalmovement between the respective wheel and the body. Each front wheel ramis connected to the diagonally opposite rear wheel ram by a respectivepair of fluid communicating conduits a first one of said pair ofconduits connecting the first chamber of the front wheel ram to thesecond chamber of rear wheel ram and the second one of said pair ofconduits connecting the second chamber of the front wheel ram to thefirst chamber of the rear wheel ram. Each pair of conduits and the frontand rear wheel rams interconnected thereby constituting a respectiveclosed circuit whereby first and second closed circuits are formed, anda pressure distribution means interposed between the first and secondclosed circuits and adapted to substantially achieve pressureequilibrium between said closed circuits, said pressure distributionmeans comprising two primary pressure chambers, each divided into twosecondary pressure chambers by piston means, the piston means of saidprimary chambers being operatively interconnected to transfer motiontherebetween, and permit controlled independent motion to vary therelative position of the piston means in said primary pressure chamber,said controlled independent movement maintaining said substantialpressure equilibrium and permitting additional controlled pitchresilience.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood from the followingdescription of a number of alternative arrangements of the vehiclesuspension system with reference to the accompanying drawings.

In the drawings,

FIG. 1 is a diagrammatic representation of the suspension system.

FIG. 2 is an enlarged diagrammatic view of the load distribution unit asincorporated in the suspension system shown in FIG. 1.

FIGS. 3 to 7B illustrate alternative forms of the load distribution unitthat may be used in the suspension system shown in FIG. 1.

FIGS. 8 and 9 are diagrammatic representations of a vehicle with aversion of the suspension system incorporating single acting hydraulicrams to support the vehicle body.

FIG. 10 is a diagrammatic representation of a suspension systemincorporating a further alternative load distribution unit.

FIG. 11 is a suspension system as shown in FIG. 1 incorporating a loaddistribution lockout arrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, four hydraulic cylinders or rams 1, 2, 3, and 4are located in between the vehicular body/chassis (not shown) and thewheel units (not shown) so that as each wheel moves relatively to thechassis, the rams are caused to be contracted or extended.

As shown in FIG. 1, the ram is functionally related to the front leftwheel while ram 2 is similarly associated with the front right wheel.Ram 3 is associated with the rear right hand side wheel while ram 4 islocated between the rear left wheel and the chassis. The front of thevehicle is therefore represented towards the top of the page.

Four conventional oil over gas accumulator springs 5, 6, 7, 8 are shownsuch that accumulator 5 is associated with the front left wheel andaccumulator 8 with the rear left wheel, for example. The portion orchamber 5a, 6a, 7a, 8a of each of the accumulators are filled with gas,while the hydraulic oil filled portions 5b, 6b, 7b, 8b are incommunication with the gas chambers 5a,6a,7a,8a. The oil and gaschambers are normally separated with a flexible diaphragm or freepiston. Damper valves 5c, 6c, 7c, 8c are conveniently located in themouths of the accumulators 5, 6, 7, 8.

When double acting wheel rams are used, they are normally conventionallydivided into two reciprocal chambers including a larger chamber 1a, 2a,3a, 4a, and a smaller chamber 1b, 2b, 3b, 4b, the smaller chamberaccommodating a piston rod.

The upper larger chamber 1a is connected to the lower smaller chamber 3bof the diagonally opposite wheel by way of pipe or conduit 9 while theupper chamber 3a of this wheel, is connected to the lower chamber 1b ofthe first cylinder by way of pipe 10. These pipes therefore complete apair of fluid circuits interconnecting one pair of diagonally oppositewheels, top to bottom chamber and visa versa. The other pair ofdiagonally opposite wheels are similarly interconnected 2a to 4b viaconduit 11 and 4a to 2b via conduit 12.

Located (centrally) in any convenient place and in any suitable mannerthere is a component which can be referred to as the load distributionunit 13.

An earlier version of a load distribution unit is described in theApplicants' earlier patent WO 93/01948 and is normally constructed outof cylindrical tube divided by a fixed wall and having a movable pistonin each chamber.

The two pistons in the former proposed construction were directlyconnected by a rod extending through the entire length of the cylinderso that both pistons were caused to move together in unison resulting intwo of the chambers becoming enlarged simultaneously while the other twowere being caused to contract reciprocally at the same time. This designhas been found to lead to some inherent problems in certaincircumstances described as follows.

In the previously disclosed construction, the piston, and rod assemblyis unable to move in response to two orthogonal wheel inputs (such asmovements over parallel speed humps). The chambers of the loaddistribution unit were described as being hydraulically connected to thewheels in a sequence such that the fluid pressure and volume changesresulting from two wheel orthogonal inputs oppose each other in order toprevent piston movements within the load distribution unit. The originalproposed system was designed specifically to only permit piston and rodmovements in the load distribution unit as a response to diagonal axlearticulations which induce fluid pressure and volume changes in the loaddistribution unit to ensure optimal weight was being born by each of thefour wheels without regard to their wheel travel positions.

As a consequence of the restriction of movement of the piston and rodassembly in response to two orthogonal wheel inputs suspension, pitchingmotion becomes noticeable in specific situations.

When, for example, the front pair of wheels encounters an obstacle, suchas a speed hump across the width of the road, the fluid is firstlyexpelled from the top chambers of both the front rams, and some of thisfluid is forced to enter the associated accumulators via damper valves.Generally the greater the resistance offered by these damper valves thegreater the volume of fluid there will be forced to other parts of theconnected hydraulic system, and inevitably some fluid enters thediagonally opposite lower chambers.

The transfer of fluid volume from the front rams into the lower chambersof the rear rams forces the associated pistons upwards therebycontracting the two rear rams, which in turn-tends to cause the rear ofthe vehicle to squat. The delay in this procedure at some frequenciescan cause the rear of the vehicle to still be moving downwards orsquatting when the rear pair of wheels encounter the same speed hump andthis can lead to a rapid pitching response as a very rapid change indirection is required at the rear of the vehicle. The additional impacton the wheels of the rear axle also then further contracts the rear ramsand this additionally compresses the gas in the associated accumulators.

As the rear wheels depart from the downside of the speed hump thecumulative compression of the gas in the rear accumulator gas chambercaused by the quick succession of front and rear wheel inputs ispermitted to expand and thereby expels fluid from the rear accumulatorfluid chambers. This can then cause the rear rams to overextend causingthe rear of the vehicle to lift beyond the level which is required tore-establish normal ride height. If more humps in the road are thenencountered by the front wheels before the vehicle has settled, aresonant response can be set up and can become exaggerated as rapidreversals in vehicle motion take place.

These motions can be at least partially negated by the dampers in themouths of the accumulators as well as the restrictors located in theconduits. Nevertheless, various permutations of uncomfortable responsesto poor road surfaces can occur depending on wheel base length, distancebetween bumps, speed, damping rates, spring rates and physical locationof the rams with regard to the wheel geometry, for example.

The load distribution unit 13 according to the present inventionprovides resilience to lessen/suppress high frequency small amplitudeinputs, and also provides some additional resilience in either pitch orroll motion specifically. The load distribution unit 13 has similaritiesin construction to the above described unit in that there are providedfour chambers. However the one piece piston rod referred to in the priorspecifiction is replaced with two piston rods with a resilient bufferinterconnecting the two piston rods. FIGS. 1 to 11 show variousalternative constructions of the load distribution unit 13 which are allgenerally divided into two cylinder portions 13a, 13b each of whichcomprise two reciprocal volume chambers, 14, 15, 16, 17. The samereference numerals are used in each of the Figures for correspondingcomponents.

In FIG. 1 and 2, the load distribution unit 13 is shown with four mainchambers 14, 15, 16, 17 respectively in direct fluid communication withbranch lines 9a, 12a, 10a, 11a respectively and these are in fluidcommunication with ram chambers 1a, 3b, and 4a, 2b, and 3a, 1b, and 2a,4b respectively via conduits 9, 12, 10, 11.

Chambers 14 and 15 within one cylinder portion 13a act reciprocallytherein, as do chambers 16 and 17 in the other cylinder portion 13b.Each cylinder portion 13a, 13b supports a piston assembly 18,19, eachpiston assembly having a piston 18d,19d, an outer piston rod portion18a, 19a and an inner piston rod portion 18b, 19b. The two outer pistonrod portions 18a, 19a normally terminate outside the chambers at bothends of each of the cylinders to enable the pistons to move freely withrespect to the cylinders.

The ends of the inner piston rod portions 18b, 19b which typically faceeach other may be provided with any convenient attachment means such asthe disc fittings shown at the rod ends numbered 18c and 19c.

Between the opposed piston rod assemblies, a resilient member or buffer20 is introduced to provide resilience in either compression or tensionor both. In the example shown in FIG. 2, the resilient member 20 maycomprise a rubberised portion which is joined or bonded to the discs 18cand 19c in FIG. 2.

Any movement therefore induced by fluid pressure and volume changes inone pair of reciprocal chambers (such as chambers 14, 15 in cylinderportion 13a) is indirectly transferred via the rod 18 and through theresilient member 20 into the other cylinder portion 13b of the loaddistribution unit 13 via rod 19 and thereby into the other pair ofchambers 16, 17. The purpose of this indirect connection/couplingbetween the two cylinder portions may be described as follows:

It will be seen in FIG. 1 (in conjunction with FIG. 2) that the upperchambers 1a, 2a of the rams associated with the front wheels are influid communication with chambers 14 and 17 within the opposed cylinderportions of the load equalisation unit 13. If an obstacle (such as aspeed hump) is encountered by both front wheels simultaneously, fluidwill become expelled out of the upper chambers 1a and 2a.

Some fluid will initially enter the nearest accumulators 5 and 6 throughdamper valves 5c and 6c and some may be distributed to the rear ram andthe control unit. Some fluid under increased pressure will thereforeenter the branch lines 9a and 11a associated with the top front chambersof the rams 1 and 2. This fluid then enters chambers 14 and 17 at theopposite ends of the distribution unit and urge these chambers toenlarge. As they enlarge in volume the two piston and rod assemblies 18,19 are forced to slide towards each other and this compresses theresilient member 20 which is located between the two cylinder portions13a and 13b

As the two piston assemblies 18, 19 are forced to move towards oneanother the chambers 15, 16 (which are reciprocal with chambers 18, 19respectively) become progressively diminished in size and expel fluiddown branch lines 12a, and 10a, into conduits 12 and 10 and therebyintroduce fluid at a slightly greater pressure into cylinder chambers2b, 4a, and 1b, 3a. This has the effect of further softening the impactof the speed bump on the front axle by pushing up the pistons withinrams 1, and 2, and more importantly it provides fluid to the topchambers 3a, 4a of the back rams which tends to raise the rear of thevehicle up as the rams are extended in preparation for the rear wheelsimpacting the same speed humps.

It should therefore be noted that the resilient member 20 therebylargely reverses the adverse pitch response in the longitudinal plane ofthe vehicle relative to the prior proposed construction and this thensoftens the pitch harshness and helps to stabilise resonant pitchingmotions.

It should also be understood that while the configuration described withreference to FIGS. 1 and 2 modifies and softens pitch motions it doesnot effect roll stiffness. However, if it was required that rollstiffness was to be reduced instead of pitch stiffness the branch lineconduits need only be exchanged to connect to the appropriate differentchambers on the load distribution unit.

A further benefit of introducing a resilient member 20 into the loaddistribution unit is that the vehicle's general softness and comfort canbe enhanced by the introduction of the resilient member 20 withoutsacrificing roll stability. It is also possible to reduce the amount ofgas in the accumulators below what would normally be required so thatthe reduction in gas volume adds roll stiffness without adverselyeffecting comfort levels. Additionally roll stability does not reduce asmore weight is loaded onto the vehicle as the gas in the accumulatorsbecomes more compressed which effectively reduces roll.

The introduction of a resilient member such as the rubber component 20,however, can provide some dynamic isolation between wheels when fastersmall axle articulation movements are occurring. As large axlearticulation movements occur only when driving at very slow speeds suchas when operating off road, the consequence is there are few pressurespikes energising the resilient member 20 which causes dynamicisolation, and therefore, during slow articulations such dynamicisolation is not noticeable.

The softness of the resilient member 20 should be such that the twopiston assemblies 18, 19 should substantially follow each other (or pushand pull one another without there being much movement difference orloss between the two piston assemblies 18, 19) when there are single orslow diagonally opposed wheel inputs occurring such as when axlearticulation is taking place, but the resilient member should not be sohard that when two wheels on the same axle encounter a sudden bump ordepression simultaneously that the resilient member is not readilydeformed to enable a delayed response in adverse pitch motions.

In practice, pressure spikes are significantly greater during two wheelhigh speed inputs than during slow speed articulation when somestiffness is required so there is some latitude with the choice ofresilient means 20. In this context, the resilient member can optionallybe replaced with any suitable damping mechanism which can similarlydelay the transfer of force and movement from one shaft to the otherwithout there being a resilient member such as a spring between theshafts.

In the FIGS. 3 to 7, the resilient means are illustrated in differentforms, such as rubber or urethane blocks, coil springs, or gasaccumulator types. It should be understood that within the scope of thisinvention that disc springs and other resilient means may equally beused and the spring mechanisms only serve to return the component partsto their correct relative positions.

In this regard the two cylinder portions 13a, 13b of the loaddistribution unit 13 are understood to be mechanically attached to oneanother so that the relative motions of their piston assemblies 18 and19 do not cause their housing cylinders to also move. The attachmentcleats numbered 21a, 21b, 13c, 13d therefore illustrate attachment meansto the chassis (or any convenient member) of the two cylinder portions13a, and 13b respectively.

The resilient means 20 is normally either held in compression or tensiondepending on which fluid conduits are connected to which chambers inrelation to which end (or side) of the vehicle is heaviest at any giventime and also with regard to the relative sizes of the rams 1, 2, 3, 4bores and rods which define the relative system pressures. It istherefore necessary to design all components relatively so that theresilient member 20 is given the appropriate spring rate orhardness/durometer rating to compensate for any bias and/or expectedweight variations.

It should be noted that as the resilient member 20 is frequently aspring of some nature it can become beneficial to introduce a dampingcomponent into the load distribution unit to damp out any unwantedspring resonance within this unit. The damping means may be designedinto the load distribution unit as an integral part within the body ofthe unit. Alternatively the two ends of a damper (such as the telescopicshock absorber numbered 22 in FIG. 2) may be attached to the two rodends (as at 18a and 19a) so that the damper is extended and contractedin direct response to any movement induced by two orthogonal wheelinputs but not by diagonal two wheel inputs such as when articulation isoccurring. This further ensures that damping occurs specifically andonly when needed and that the frictional resistance is minimised by thedamper during axle articulation. This is important so that optimal evenground pressure occurs at the wheels during axle articulation whileadditional damping occurs when wheels impact parallel obstacles withsuccessive axles.

The damper unit should be regarded as an important optional component asit permits the tuning of specific functions in the suspension system.The damper also can be used to delay the responses and interactionsbetween the front and back axle so that inputs at sensitive frequenciesresulting from wheel base length road conditions do not upset vehiclessmooth passage. Dampers may also take the form of (optionally variable)restrictors 9b, 9c, 12b, 12c, 11b, 11c, 10b, 10c within the conduits,which permit the individual tuning of the various components. Forexample, when the restrictor-dampers 9b, 10b, 11b, 12b are introduced,fluid is restricted from communicating with the lower chambers 1b, 2b,3b, 4b so that the resilient effects of the load distribution unit 13are maximised. Conversely, when the dampers 9c, 10c, 11c, 12c are mainlyused this prevents the free communication of fluid from the rams to theload distribution unit and encourages fluid to act upon the lower ramchambers 1b, 2b, 3b, 4b with very different consequences. Adjusting thebalance of the restrictions exerted by restrictors 9b, 10b, 11b, 12bwith reference to restrictors 9c, 10c, 11c, 12c provides the ability toallow for the appropriate tuning of the total damping forces acting onthe vehicle. Such tuning can also be accomplished through the carefulselection of conduit sizes to provide the appropriate amount of frictionto arrive at similar damping responses.

FIG. 3 is another alternative method of construction of the loaddistribution unit 13. In this embodiment the two cylinder portions 13a,13b are remotely coupled by the resilient means 20a, which may comprisea cylindrical portion 19c fixed to rod portion 19b in the place offlange 19c in FIG. 2. At the end of the cylindrical portion facing theother rod there is a hole which can easily accommodate the opposed rodportion 18b.

The end of the opposite rod portion 18b extends through the hole incylinder 19c so that a flange 18c provided on the end of the rod portion18b is located inside the chamber and towards the centre of the cylinder19c. On either side of the flange 18c there is provided a resilientmeans such as a coil or disc spring or rubber block numbered 18e, 19e.Alternatively the chambers on either side of the flange or piston 18cmay be charged with gas to provide a gas spring to remotely locate thepiston 18c within cylinder 19d.

One advantage of the resilient means such as 20a over 20 is that the tworesilient members may be individually constructed differently to bestsuit their required functions as tension or compression members withreference to the other parts of the suspension system.

Resilient member 20a may alternatively be constructed by using a shockabsorber (damper unit) provided with one or two concentric coil springsinternally or externally of the telescopic component.

FIG. 4 shows another alternative to the central resilient member of FIG.2. This version differs from that shown in FIG. 2 in that the resilientmember 20 is kept in compression within half cylinders 18c, 19cregardless of whether the two half cylinders 18c and 19c are moving awayfrom one another or towards each other. By keeping the resilient meansin compression, the mechanical problems of bonding the rubber blocks tothe end flanges 18c, and 19c of the embodiment of FIG. 2 are obviated.

In FIG. 5, a gas charged version of the resilient member is illustrated.In essence the two rod ends 18c, 19c are constructed as pistons bearingseals within the cylindrical chamber 21 which is an extension of thecylinder portions 13a, 13b. The two pistons subdivide the cylinder 21into three minor chambers 21a, 21b, and 21c as shown.

Chambers 21b and 21c, on either side of the central chamber 21 a areinterconnected by way of conduit 21c so that these two chambers remainsubstantially at the same pressure whilst still remaining reciprocal involume. The purpose of this is to prevent a bias developing which wouldsubstantially centre pistons 18c, 19c thereby restricting axlearticulation with even wheel loading.

Two gas charge valves 22a, 22b are provided so that one valve 22aenables chamber 21a to be charged to an appropriate pressure to resistthe pressure differential caused by the front of the vehicles weightexceeding that of the back, and valve 22b enables chambers 21b and 21cto be jointly charged to provide resilience sufficient to maintainvehicle height when the vehicle may weigh more at the rear as during apitch motion.

FIG. 6 is yet another version of the central resilient means andrepresents another gas charged form of the resilient means.

In this instance, the two cylinder portions 13a and 13b are locatedparallel so that their ends 18c and 19c do not face each other but facein the same direction. The central cylindrical chamber 21 is divided sothat one half is located adjacent one cylinder portion 13a andaccommodates piston 18c, and the other half of cylinder 21 locatedadjacent the other cylinder portion 13b and accommodates piston 19c.

Chambers 21b and 21c are linked by conduit 21d as in FIG. 6 so that theymaintain a substantially equivalent pressure. As chamber 21a is nowdivided into two sections 21a(i) and 21a(ii) these are now similarlylinked by way of conduit 21e.

In function, the version shown in FIG. 6 is the same as that shown inFIG. 5 but the advantage of the version in FIG. 6 is that the overalllength of the load distribution unit 13 is reduced which facilitatespackaging.

FIGS. 7a and 7b shows another version of the load distribution unit inwhich the two cylinder portions 13a and 13b are located in parallel. Inthis version however, the resilient gas spring chambers are replacedwith rubber blocks or coil springs in the following way;

FIG. 7a represents a similar elevation view equivalent to that shown inFIG. 6. FIG. 7b is another elevation diagram drawn at right angles tothe first and is included for clarity.

Inner piston rod portions 18b, 19b are elongated to extend to points18d, 19d. At some point along the length of the extended rod portionsthere are points 18c, 19c which are equivalent in function to the partsnumbered 18c and 19c in the other figures. In these FIGS., 18c and 19cmay typically comprise a disc mounted and slidably located on the rods18b and 19b. Protruding from each of these two discs on opposite sidesthere are two spigots or small rods which carry four arms 18e, 19e.These in turn are similarly flexibly joined at points 18f, 19f to acommon rocker arm 23 which is pivotally mounted to the same member asthat which locates the rest of the body of the load distribution unit13. Disc and spigot units 18c and 19c are therefore mechanically andreciprocally mounted with respect to each other so that if one moves`up` the other moves `down`.

Resilient members 24a and 24b are functionally similar to the resilientmeans 18d 19d in FIG. 3 or the gas springs 21a (i) and (ii) and 21b andc, in FIG. 6, for example. In the example shown in FIG. 7 the resilientmeans 24a and b may be rubber or urethane blocks concentrically locatedaround rods 18b and 19b and held between end stops marked 25a and 25bwhich are prevented from moving apart on the rods by any convenientmeans as shown.

If therefore, the vehicle wheels impact a speed hump with one axle, rodportions 18a and 19a will both be caused to be thrust downwards (withreference to the drawing) which would cause both sets of rubber blocks24a to become compressed between discs 25a and 18c, 19c while the otherrubber blocks 24b are permitted to extend. The impact of the two frontwheels would therefore be borne to a degree by the blocks 24a and asimilar impact on the two rear wheels simultaneous would cause thecompression of the rubber blocks 24b to bear some of the impact.

If however the impact is solely on diagonally opposite wheels as duringaxle articulation, the four rubber blocks would remain substantiallyundistorted while one piston rod may be extending one way the other iscontracting in the opposite direction. In this way, load distribution isoptimally maintained during diagonal wheel movements while twoorthogonal wheel inputs are partially resolved by the resilient means24, and while roll forces on the other two orthogonally disposed wheelsare resisted hydraulically.

Referring again to FIG. 1, at some point along the length of eachconduit there may be optionally located a fixed or adjustable valve tovary the degree of resistance to the flow of fluid through the conduits.These valves are marked 9b, 10b, 11b, 12b and are normally locatedbetween the smaller cylinder chambers and the branch lines 9a, 10a, 11a,12a, which in turn may have further restrictors 9c, 10c, 11c, 12clocated along their lengths. Normally in operation these valves permit alarge volume of fluid to flow at low speed (as during axlearticulation), while the valves restrict the flow of smaller volumes offluid at higher speeds which are typical of wheels impacting bumps atspeed and which tend to upset the smooth running of the vehicle.

Additionally, it is found that for packaging reasons it is sometimespreferable to have as few accumulators in the wheel arch areas aspossible and accordingly only one accumulator per hydraulic circuit isindicated although maximum comfort may be obtained by the inclusion of asecond small accumulator located near the lower chambers 1b, 2b, 3b, 4b.Moreover, with reference to the type of layout shown in FIGS. 1 to 7(which permit additional pitch resilience as a direct result of thedesign of the resilient means within the load equalisation units 13) ithas been found that only a small gas volume is normally required in theaccumulator associated with the lower chambers of the hydrauliccylinders.

FIG. 8 of the drawings depicts the present invention applied to avehicle wherein the hydraulic rams 1, 2, 3 and 4 are single acting ramsin contrast to the double acting rams as described with reference toFIG. 1. As a result of the rams being only single acting, each of theconduits 9, 10, 11 and 12 only connect the respective upper chambers 1a,3a, 2a and 4a of the rams with chambers 14, 16, 15 and 17 respectivelyof the load distribution unit 13. Thus, the portions of the conduitswhich communicated with the lower chambers 3b, 1b, 4b and 2b of thehydraulic rams can be eliminated.

Flow restrictors 9b, 10b, 11b and 12b positioned in the remainingportions of the conduits 9, 10, 11 and 12 may be desirable to allowfurther tuning of the suspension characteristics. Variable flowrestrictors can be fitted to a `semi-active` evolution of the system.

FIG. 8 also illustrates further modifications to the load distributionunit 13 which may optionally be included in the version shown in FIG. 1.In FIG. 8 the outer rod portions 18a and 19a are now of differingdiameters to the inner rod portions 18b and 19b. This may be necessaryto create differential areas from one side of the piston to the other inorder to compensate for differential system pressures from front to reardue for example to uneven vehicle weight distribution. The outer rodportions could be larger or smaller in diameter compared to the innerrod portions depending on the direction of the bias, which in turn isdictated by the connection sequence of the conduits to the loaddistribution unit.

FIG. 9 shows the same circuit arrangement as depicted in FIG. 8 but withthe modifications being restricted to the load distribution unit 13.Whereas in FIG. 8, outer rod portions 18a and 19a were of differingdiameters to inner rod portions 18b and 19b, by the same reasoning theouter rod portions 18a and 19a could be omitted completely if desired.Depending on the weight distribution of a vehicle it may be necessary tochange the sequence in which the conduits are connected to the loaddistribution unit as mentioned above.

In addition, the load distribution unit 13 as shown in FIGS. 1 and 2 hasbeen modified in FIG. 9 by replacing the pistons 18 and 19 in FIG. 2with a piston constructed in essentially the same way as resilientmember 20 as seen in FIG. 2. Equivalents of the disk sections 18c and19c sandwiching the resilient member 20 in FIG. 2 are constructed tofunction as pistons which reciprocate in the chambers 13a and 13b in thesame basic manner as the single pistons as shown in FIGS. 1 and 2.However the use of this construction of the pistons, having theintermediate resilient section, results in some limited resilientmovement between the two sections of the piston when one of the wheelsis subjected to a sudden shock loading, in the same manner as the loaddistribution unit 20 does as previously described with reference toFIGS. 1 and 2. Movement during roll would also be induced resulting in areduction in roll stiffness with the configuration shown in FIGS. 8 and9.

FIG. 10 shows a further preferred alternative form of the loaddistribution unit 13. In addition to chambers 14, 15, 16, 17 provided inthe aforementioned forms of the distribution unit 13, the two pistonassemblies 18, 19 are separated by a centre chamber 35 containing acompressible gas or a fluid. An accumulator 38 communicates with thecentre cylinder 35 and movement of the piston assemblies 18, 19 towardseach other will be resisted by the gas or fluid contained within thecentre chamber 35. The outer rod portions 18a, 19a are larger indiameter than the inner rod portions 18b, 19b and are respectivelyaccommodated within outer chambers 33, 34. These outer chambers areconnected by a conduit 36, with a further accumulator 37 being providedon this conduit 36. Fluid is contained within the outer chambers 33, 34and connecting conduit 36, and movement of the piston assemblies 18, 19away from each other is resisted by the fluid contained therein.

This load distribution unit 13 has the ability to control pitch andaccommodate large variations in vehicle load. For example, if asignificant load is added to the rear of the vehicle, the pistonassemblies 18, 19 of the load distribution unit 13 will be urged furtherapart due to the increased pressure and fluid volume in the innerchambers 15, 16 thereof. To compensate for the increased load from theinner chambers 15, 16, additional fluid may be introduced into outerchambers 33, 34 by a pump 40 or other means to increase the pressureacting on the ends of the outer rod portions 18a, 19a thereby allowingthe pistons 18, 19 to return to their correct operating positionsdespite the increased load in the vehicle. Conversely, when the pistonassemblies move too close together, it may be necessary to release fluidfrom the outer chambers 33, 34 to a tank 41 to compensate for removingload from the vehicle or for a load added to the front of the vehicle.Fluid may also be pumped or drained from the centre chamber 35 tocontrol the position of the piston assemblies 18, 19. Returning thepistons 18,19 to their correct operating position allows greaterclearance for movement of the piston assemblies to thereby prevent anyrestriction of the movement of the pistons 18,19 within their respectivecylinder portions. Therefore, for a set pressure in the centre chamber35 (ideally achieved through the use of a pressure regulator valve) theload distribution unit 13 may be controlled to compensate for changes inthe load in the vehicle.

To control the necessary flow of fluid to and from the alternative loaddistribution unit 13, a load distribution unit position sensor(preferably a Hall Effect sensor) is required to enable the position ofeach piston 18, 19 to be ascertained. In order to achieve the correctpositioning of said pistons, an electronic control unit averages theload distribution unit piston position sensor signals to attain thedesired initial spacing between the pistons 18, 19 by supplying orreleasing fluid from the outer chambers 33, 34.

Further details of this load distribution unit is disclosed in theApplicant's international application No. PCT/AU94/00646, and detailsare incorporated herein by reference.

As noted above, the suspension system according to the present inventiontends to reduce pitch motions of a vehicle when going over speed humpsor other obstacles. It is however preferable to maintain the height ofthe rear of the vehicle up until immediately before the rear wheelsimpact the speed hump and to retract the rear rams as the rear wheelstravel over the hump. This helps to further reduce the pitch motion ofthe vehicle as it travels over humps or other obstacles.

To this end, a fast acting valve 42, such as a solenoid valve, isprovided at the mouth of one or both the accumulators 37, 38 of the loaddistribution unit 13. This valve 42 can for example be provided at themouth of the accumulator 38 of the centre chamber 35 as shown in FIG.10. As gas or fluid enters the accumulator 38 when the front wheels hita hump and fluid is expelled from the top chambers 1a, 2a of the frontrams 1, 2 to the load distribution unit 13, the solenoid valve 42 cantemporarily close off the accumulator 38 to therefore store thepressurised gas or fluid. An electronic control unit can determine whenthe rear wheels impact the hump and this has the effect of retractingthe rear rams 3, 4 as the rear wheels travel over the hump so that thepitch motion of the vehicle is further reduces the input to the rearaxle.

FIG. 11 shows the suspension system of FIG. 1 which has been modified toincorporate a "load distribution lockout" arrangement.

The Applicant's suspension system has the advantage in allowing largedegrees of axle "articulation" without significantly affecting thenormal reaction force at each vehicle wheel to the ground therebymaintaining a similar amount of traction over extremely rough ground asover flat ground. The term "articulation" refers to the movement ofdiagonally disposed wheels in a common direction. Furthermore, thesesystems oppose and thereby limit the body roll motions created when thevehicle is cornering without the need for roll stabiliser bars.

It has been found that in vehicles fitted with the above notedsuspension system, during extreme situations, for example, when there isa combination of very fast cornering and hard braking or fastacceleration, this can result in the lightly loaded wheel of the vehiclelifting completely off the ground. While this does not necessarilyeffect the overall stability of the vehicle, the lifting of one of thevehicle wheels can be disconcerting.

The load distribution lockout arrangement includes at least one"lock-out" valve 30a, 30b provided on at least one of the branchconduits 9a, 10a, 11a, 12a. In the arrangement shown in FIG. 11, valves30a, 30b are respectively provided in branch conduits 9a and 11acommunicating with the outer chambers 14, 17 of the load distributionunit 13.

When the valves 30a, 30b are actuated to block the flow of fluid throughthe branch conduits, this acts to disable the suspension system so thatarticulation in the suspension system is restricted or prevented. Thisstops or minimises the lifting of the wheel under the above notedextreme driving conditions. These conditions can be sensed via a sensormeans mounted on the vehicle, the sensor means for actuating the valves30a, 30b.

The sensor means may include an acceleration sensor. Alternatively or inaddition, the sensor means may include a vehicle speed sensor. Thesensor means may provide a signal to the control means when both lateraland longitudinal accelerations of the vehicle in excess of programmablepreset levels are detected simultaneously, the control means therebyactuating the locking means. The control means may only actuate thelocking means when the signal from the vehicle speed sensor indicatesthe speed to be above a preset level. This prevents actuation of thelocking means when the vehicle is traversing rough terrain.

It should be noted that the sensor means may comprise many differenttypes of sensors as long as the control means is able to determine fromthe available inputs the appropriate reactions to the lateral andlongitudinal accelerations acting on the vehicle. For example, thesensor means may alternatively consist of speed, steering angle,throttle position and brake pedal position sensors. By using resilientmembers 20 in place of pistons in the load distribution unit 13 as shownin FIG. 9, it is possible to eliminate the need for accumulators 5, 6,7, 8 in the suspension system. This will generally be the case for anyof the embodiments of the suspension system according to the presentinvention.

The load distribution unit according to the present invention providesthe suspension system with additional resilience in the pitch directionof the vehicle while not increasing compliance of the vehicle in theroll direction. This has the advantage in that the front wheels arepractically "de-coupled" from the back wheels when the front wheelsimpact a speed hump or other obstacles so that the back wheels are notsignificantly influenced by the movement of the front wheels in thissituation. This results in less harsh pitch motions of the vehicle andsofter ride.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

We claim:
 1. A suspension system for a vehicle, the vehicle having abody and at least one front and one rear pair of wheels, the body beingsupported above each wheel by a respective double acting ram, the doubleacting rams being interconnected between the wheel and the body;eachdouble acting ram comprising a cylinder bore separated into a first anda second fluid filled chamber by a piston, a cylinder rod being fixed tothe piston and extending through one of the first or second fluid filledchambers; each front wheel ram being connected to the diagonallyopposite rear wheel ram by a respective pair of fluid communicatingconduits, a first conduit of each said pair of fluid communicatingconduits connecting the first chamber of the front wheel ram to thesecond chamber of the rear wheel ram, and a second conduit of each saidpair of fluid communicating conduits connecting the second chamber ofthe front wheel ram to the first chamber of the rear wheel ram, eachsaid pair of conduits and the front and rear wheel rams interconnectedthereby constituting a respective circuit whereby first and seconddiagonal circuits are formed; a pressure distribution means interposedbetween the first and second circuits and adapted to substantiallyachieve pressure equilibrium between said circuits, said pressuredistribution means including two primary chambers, each primary chamberincluding two system chambers defined by respective movable walls, thetwo moveable walls being interconnected from one said primary chamber tothe other said primary chamber by force transfer means; wherein theforce transfer means includes a resilient means to permit relativemotion between the moveable walls and thereby provide additional pitchresilience in the suspension system.
 2. A suspension system as claimedin claim 1, wherein the system chambers of one primary chamber areconnected to the first chambers of the double acting rams on one side ofthe vehicle, the first chambers of the double acting rams on theopposite side of the vehicle being connected to the system chambers ofthe other primary chamber, such that roll motions of the vehicle areresisted by the pressure distribution means;both of the first and seconddiagonal circuits being connected to the pressure distribution meanssuch that the first conduit of each said diagonal circuit is connectedto a said system chamber in one said primary chamber, the second conduitof said circuit being connected to a said system chamber in the othersaid primary chamber such that the pressure distribution meanssubstantially achieves pressure equilibrium between said first andsecond diagonal circuits, thereby providing the suspension system withnegligible resistance to cross-axle articulation motions; and thedouble-acting rams at one end of the vehicle are connected to thepressure distribution means such that the first chamber of the ram onone side of the vehicle is connected to a said system chamber in onesaid primary chamber and the first chamber of the ram on the other sideof the vehicle is connected to an opposing said system chamber in theother primary chamber, such that the resilient means in the forcetransfer means provides additional pitch resilience in the suspensionsystem.
 3. A suspension system as claimed in claim 2, wherein theresilient means is effected by a resilient member.
 4. A suspensionsystem as claimed in claim 2, wherein the resilient means is effected bya compressible gas means.
 5. A suspension system as claimed in claim 2,wherein said force transfer means includes respective piston rodsprojecting from each moveable wall, the rods at adjacent ends extendinginto a common chamber isolated from the primary chambers, said commonchamber being charged with a fluid to apply equal force to the pistonrod of each moveable wall.
 6. A suspension system as claimed in claim 5wherein an accumulator is in operable communication with said commonchamber for fluid flow therebetween.
 7. A suspension system as claimedin claim 6 wherein the operable communication is through a selectivelyvariable flow rate passage.
 8. A suspension system as claimed in claim5, wherein said force transfer means includes further piston rodsprojecting from the moveable walls, and a respective third chamber inwhich each rod extends, said third chambers being respectively locatedon the opposite side of the primary chambers to the location of thecommon chamber, said third chambers being in operable communication forfluid flow therebetween.
 9. A suspension system as claim in claim 8wherein said operable communication includes an accumulator for fluidflow therebetween.
 10. A suspension system as claimed in claim 8,wherein means are provided to selectively supply or withdraw fluid fromsaid third chambers.
 11. A suspension system as claimed in claim 10,wherein means are provided to selectively supply or withdraw fluid fromsaid common chamber.
 12. A suspension system as claimed in claim 8,wherein the diameter of said piston rods is different to the diameter ofsaid further piston rods.
 13. A suspension system as claimed in claim 5,wherein the fluid in the common chamber is a gas.
 14. A suspensionsystem as claimed in claim 5, wherein means are provided to selectivelysupply or withdraw fluid from said common chamber.
 15. A suspensionsystem, as claimed in claim 1, wherein means are provided to selectivelyisolate the pressure distribution means from at least one of said fluidcommunicating conduits of the diagonal circuits.
 16. A suspension systemas claimed in claim 1, wherein said force transfer means includes arespective rigid member projecting from each moveable wall into afurther chamber and attached to respective control pistons therein, saidcontrol pistons defining within said further chamber a first controlchamber between said two control pistons and on the opposite side ofeach control piston respective second control chambers, said first andsaid second control chambers each being charged with a fluid to normallycentralize the pistons defining the system chambers and to permitcontrolled movement therebetween to thereby permit independent controlof the pitch and roll of the vehicle.
 17. A suspension system as claimedin claim 16, wherein the second control chambers are disposed in a sideby side relation and said control pistons are each disposed inrespective side by side chambers.
 18. A suspension system as claimed inclaim 16, wherein the fluid in the control chambers is a gas.
 19. Asuspension system as claimed in claim 1, including damping means fordamping said relative motion between the moveable walls.
 20. Asuspension system for a vehicle, the vehicle having a body and at leastone forward and one rear pair of wheels, the body being supported aboveeach wheel by a respective double acting ram, the double acting ramsbeing interconnected between the wheel and the body;each double actingram comprising a cylinder bore separated into a first and a second fluidfilled chamber by a piston, a cylinder rod being fixed to the piston andextending through an end of the second fluid filled chambers; each frontwheel ram being connected to the diagonally opposite rear wheel ram by arespective pair of fluid communicating conduits, a first said conduit ofeach said pair of fluid communicating conduits connecting the firstchamber of said front wheel ram to the second chamber of the rear wheelram, and a second said conduit of each said pair of fluid communicatingconduits connecting the second chamber of the front wheel ram to thefirst chamber of the rear wheel ram, each said pair of conduits and thefront and rear wheels rams interconnected thereby constituting arespective circuit whereby first and second diagonal circuits areformed; a pressure distribution means interposed between the first andsecond circuits and adapted to substantially achieve pressureequilibrium between said circuits, said pressure distribution meansincluding two primary chambers, each primary chamber including first andsecond system chambers defined by respective moveable walls; the firstsystem chamber of the first primary chamber being in fluid communicationwith the first conduit of the second diagonal circuit, the first systemchamber of the second primary chamber being in fluid communication withthe first conduit of the first diagonal circuit, the second systemchamber of the first primary chamber being in fluid communication withthe second conduit of the first diagonal circuit and the second systemchamber of the second primary chamber being in fluid communication withthe second conduit of the second diagonal circuit; the moveable wallswithin each primary chamber being functionally interconnected by a forcetransfer means incorporating a resilient means to permit resilientrelative motion between the moveable walls and thereby provideadditional pitch resilience when both said front or rear wheels arerequired to move simultaneously with additional freedom in the samedirection relative to the vehicle body and without influencing roll orwarp control.